Understanding pain has challenged philosophers, doctors and researchers for centuries. In the Far East, it began with the idea of imbalanced yin and yang forces. The ancient Greeks thought it arose from out-of-kilter bodily humors. The Muslim physician Avicenna saw it as arising from some alteration in physical condition. And, of course, numerous cultures believed that gods doled out pain as punishment and demanded it as penance [source: Encyclopaedia Britannica].
Pain research and theory began in earnest in the 19th and 20th centuries, but the medical and technological advances of the past 40 years have revolutionized the field [sources: Encyclopaedia Britannica; Craig]. A prevalent model of how pain works goes as follows.
Let's say you touch a hot stove and burn your hand. Instantly, your arm recoils and you feel pain because a network of specialized nerves called nociceptors (from the Latin noci- "harm" + receptor) has activated. Unlike other nerve types, nociceptors only trigger when they detect a harmful event, such as too much heat or pressure. When this happens, these nerves convert the noxious stimulus into an electrical signal that zips to the brain with the bad news. How? Their nerve endings change shape, creating pores that let positive ions like sodium and calcium surge in. This influx of ions drops the voltage across cell membranes and generates electrical potential. The worse the injury, the bigger the signal [sources: Wood et al.; Woolf and Ma].
That takes care of how pain nerves alert the brain and spinal cord, but how do nociceptors detect injury in the first place? In several ways, some of which we're still figuring out. Often, they detect wayward chemicals like prostaglandins. These are not "pain molecules." Rather, they are chemical substances that aid in a variety of vital bodily functions. But they should not be out bouncing around where the nociceptors can pick them up unless something has gone wrong, so they make good damage signals [sources: Ricciotti and FitzGerald; Wood et al.].
Nociceptors don't always need such chemicals to do their job; they can also detect some harmful effects directly. Excessive heat, for example, can open signal-producing ion channels on its own. So can the capsaicin in a chili pepper, which explains why, to quote Ralph Wiggum, of "The Simpsons," "it tastes like burning" [source: Wood et al.].